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The nation this month is preoccupied with memory: The shots in Dallas. The frantic drive to Parkland Hospital. The agonizing moments of uncertainty, followed by the lingering feeling of incredulity. The scene on Air Force One. The arrest and then the slaying of the accused killer. The coffin in the Capitol.

But it isn’t only memory that haunts us in the run-up to the 50th anniversary of the assassination of John F. Kennedy. We are haunted almost as much by what might have been as by what happened. What would have occurred in the remainder of the president’s term? In the 1964 election? In a second term, presuming Kennedy was re-elected?

This is more than a parlor game, though it has dominated more than one dinner from Georgetown to Moscow. It prompts us to examine how our history unfolded, whether there was an alternative course, whether the upheaval of the 1960s, the eventual election of Richard Nixon, even Watergate, might have been avoided had the bullets missed their mark, had the president survived, had the day in Dallas dawned rainy and not brilliantly bright, prompting the president to ride in a covered 1961 Presidential Lincoln rather than the open vehicle that is seared into our historical memory.

There are myriad elements to the intersection of historical forces that collided as President Kennedy’s motorcade approached the intersection of Houston and Elm streets a half-century ago. These are three of the most intriguing, troubling — and illuminating:

• The Vietnam War: Few subjects have transfixed scholars, commentators and diplomats as deeply and as persistently as the question of whether the United States would have committed a half million troops (and sent 58,000 Americans to their deaths) in Vietnam had Kennedy won a second term.

There is conflicting evidence, supporting both the notion that Kennedy would have plunged deeper into Southeast Asia and the idea that he would have withdrawn shortly after being re-elected. Both arguments point to an elemental cynicism: that the president’s fear of being blamed for losing Vietnam would have propelled him into widening the conflict, or that he would risk American lives in a half-hearted cause to preserve his political prospects and then remove troops once his own future had been secured.

Overall the better claim goes to the argument that Kennedy would have limited or ended the U.S. role in Vietnam. He and Robert F. Kennedy had visited Indochina a decade earlier and breathed the anti-colonial stench created by the French, who fled in defeat in 1954. That very year, Kennedy said on the Senate floor that “to pour money, material and men into the jungles of Indochina without at least a remote prospect of victory would be dangerously futile and self-destructive.”

Though a devout Cold Warrior, Kennedy was chastened by the Bay of Pigs and less vulnerable, as he put it, “to plunge into an irresponsible action just because a fanatical fringe in this country puts so-called national pride above national reason.”

In his “JFK’s Last Hundred Days,” published this autumn, Thurston Clarke repeatedly cites examples of Kennedy resisting entreaties to increase the American commitment to Vietnam, where, JFK told his reporter friend Charles Bartlett, the nation didn’t “have a prayer of prevailing.”

• Race: Kennedy’s evolution on civil rights outpaced the nation’s but lagged that of his Ivy League circle. Well into his administration, he was logically but not emotionally committed to equal rights and integration. But as he employed the rhetoric of “freedom” while prosecuting a Cold War abroad, he came to see the incongruity and hypocrisy in the nation’s domestic life. Eventually the sight of American authorities clashing with American citizens at lunch counters, at schoolhouse doors, at bus stations and in the streets was more than he could bear.

Some of that transition occurred during his struggle with Gov. George Wallace of Alabama, which prompted his race speech in June 1963. But some of it happened two months later, after the March on Washington. The principal organizers visited Kennedy in the White House the evening of the march. “I have a dream,” Kennedy told King in that setting, at that moment enlisting fully in King’s cause. There was no turning back.

• The youth rebellion: The Port Huron Statement, the Students for a Democratic Society manifesto written by Tom Hayden in 1962, sometimes is regarded as the clarion of 1960s upheaval, but much of the decade’s social, political and cultural tumult grew out of opposition to the Vietnam War and impatience on civil rights. So it is possible to posit that a withdrawal from Vietnam and a frontal assault on segregation might have prevented or ameliorated the rebellion of the 1960s.

Kennedy was a hero to American youths — but he had more in common with the Rat Pack than with the counter-culture. He was a libertine but did not flaunt his sexual adventurism; he acknowledged he was a combination of promiscuity and Puritanism. Even his language was antiquarian; he spoke of “dames,” not “chicks.” He was a drug abuser, but his drugs were prescription medications meant to ease his pain, not to provide a sense of ease or elevated consciousness.

Kennedy had an inherent suspicion of big business but believed in capitalism. His heroes were establishment figures like Marlborough, his blood raced to the rhythms of Kipling, his heart swung to the beat of Sinatra. He was more idealistic than iconoclastic.

Kennedy wouldn’t have thrown in his lot with the new culture — but he wouldn’t, and probably couldn’t, have stopped it. The rebellion of the 1960s might have been less rooted in protest with Kennedy alive, but the new generation, reared in the prosperity of the consumer culture and the probity of the college campus, was going to create a world that went beyond JFK, even had he lived to witness it — and be bewildered by it.